For some applications it is desirable to provide a corrosive resistant coating to objects having an intended use in a relatively hostile and/or corrosive environment. This is, e.g., the case for objects which are used as implants. To this end coatings of refractory metals, such as niobium or tantalum, are frequently used on objects made from steel or other metals or alloys. Since a coating as described above will typically have a higher corrosion potential than the substrate material positioned below the coating, the coating will only eliminate corrosion if the coating is tight. If this is not the case, e.g. because the coating contains pinholes, there is a risk of pitting. Therefore, in order to obtain the desired corrosive resistant properties by means of the coating, it is necessary to ensure that the resulting coating is at least substantially tight, i.e. that it does not comprise any pinholes. One problem in this regard is that pinholes are usually very small, and it is therefore very difficult, or even impossible, to detect them visually.
Previously, coated surfaces have been investigated in order to detect possible pinholes by inserting the object in a circuit comprising a current source and an ammeter. If a reading can be obtained on the ammeter, the coated surface is conductive and thereby tight. This method is relatively time consuming and expensive since it requires the use of a current source and an ammeter each time it is desired to investigate whether or not the coating of an object is tight.
One method of investigating a coated surface in order to visually detect the presence of possible pinholes is described in JP 57132045. In the method disclosed in JP 57132045 a coated surface of a metallic material as a cathode opposite to an anode is positioned in an electrolyte containing a solution type electrochromic material, such as a viologen dye. A voltage is applied to the cathode, thereby causing an electrolytic reduction of the viologen dye from a colourless dication I to an insoluble monocation radical II. The insoluble monocation II is deposited in pinhole parts in a red or reddish purple colour. The presence or absence of pinholes is detected with an ammeter. At the same time, the places containing pinholes present a red or reddish purple colour, and therefore their position is distinctly known. The colour remains for a while, even after removal of the voltage, and can be returned to the original state when reverse voltage is applied after the inspection.
Thus, in the method disclosed in JP 57132045 the colouring of possible pinholes is obtained by deposition in the pinholes of a dye solved in an electrolyte. The resulting change in colour will, accordingly, only affect the actual areas of the pinholes. Since pinholes are by nature very small, it will be difficult to detect them purely by vision, even if they have been visually enhanced by deposition of the dye. In addition, the need for a measurement by means of an ammeter is still required in order to detect that pinholes are present, the visual inspection merely making it easier to determine the location of the pinholes which are known to be present following the ammeter measurement. Thereby the drawbacks of the method described above are not avoided in the method of JP 57132045. Furthermore, since the change in colour is obtained by deposition of a dye in the pinholes, there is a risk that some pinholes will not be marked by the dye, thereby creating a risk that such pinholes will go undetected in a subsequent visual inspection. This is highly undesirable.